1. Field of the Invention
The invention relates to a method and a device for operating a premixing burner, in particular a burner of the double-cone type of construction which is stabilized by means of vortex breakdown, is operated in particular with gaseous fuels and is preferably used in gas-turbine combustion chambers. The device in this case relates to the fuel feed.
2. Discussion of Background
In premixing burners, such as, for example, the double-cone burner according to EP 0 321 809, the aerodynamic phenomenon of vortex breakdown is utilized in order to recirculate the hot exhaust gases and thus stabilize the fuel/air mixture for low-pollution combustion. A vortex breakdown occurs when an axially symmetrical vortex spreading forward becomes unstable and creates a backflow zone in the axis.
The premixing burners are normally designed for typical gas-turbine operating modes in such a way that their fuel/air ratio produces the least NOx emissions during operation under full load. They are therefore operated near the lean extinction limit, and their regulating range is greatly restricted.
During partial load of the gas turbine or at lower fuel feed, it is therefore necessary in order to maintain the combustion to shut off individual burners so that the remaining burners can continue to be operated in a stable manner, or the combustion mass air flow must be reduced.
An increase in the zone of flame stability would reduce the need for or the requisite accuracy of such measures and at the same time considerably increase the output of the gas turbine.
One possibility of extending the stability range of the premixing burners is the additional injection of pilot gas effected near the axis, so that the fuel gases are enriched.
To operate a burner optionally with gaseous or liquid fuel, a method is known in which the fuel oil used as an alternative to the pilot gas is atomized by injection of air near the axis of the burner. The air injection is also effected during the pilot operation with gas, but no atomization is necessary during this operation. This additional air destabilizes the pilot-gas flame and thus reduces the lean extinction limit of the flame. A method and a device for operating a combined burner for liquid and gaseous fuels have therefore been developed in which burner the atomization of the liquid fuel is effected in an airblast nozzle and the gaseous fuel in the burner interior space is enriched near the axis of the burner by feeding in pilot gas, in the case of which method and device the inflow of the blast air into the burner interior space is controlled. Thus during operation with gaseous fuel the inflow of the blast air into the burner interior space is throttled, for example by the introduction of pilot gas into the blast air.